1. Technical Field
The invention relates to an image processor, an image processing method and a computer readable medium for an image processing program, and particularly relates to the image processor performing image processing for obtaining color images and the image processing method and the computer readable medium for the image processing program applied to the image processor.
2. Related Art
The exposure when taking images is an important factor for determining quality of taken images. When taking images by setting the unsuitable exposure, there is a case in which it is difficult to discriminate a subject of the taken image because the subject is filled with black in the image though it can be seen and recognized by human eyes. Conversely, there is a case in which so-called overexposure occurs because reflected light is imaged in white in the image. Also in such case, it is sometimes difficult to discriminate a subject of the taken image.
As a related art for solving the problems and generating images having high image quality, it can be considered that plural images having different exposures and having suitable brightness are cut out and composed to form a piece of image. However, since the luminance levels of images to be composed are different in the image composite, there has been a problem that an image of a false contour appears at a boundary of composite in the image obtained by the composite.
As a related art for preventing the occurrence of the false contour, in JP-A-7-131718, when plural images having different exposures are composed, the luminance levels of plural images are allowed to correspond to one another by allowing luminance levels of images having suitable brightness (not underexposure) respectively in plural images to correspond to one another.
In the invention of JP-A-2000-78594, a circuit scale used in image composite is reduced by composite luminance of plural images before separating color.
However, image composite has a problem that it is difficult to reproduce the chroma of images appropriately in addition to the problem of the false contour. Both JP-A-7-131718 and JP-A-2000-78594 address overexposure or underexposure of images by adjusting the luminance level, which does not solve the deterioration of the chroma.
FIG. 9A and FIG. 9B are views for explaining the deterioration of the chroma in the composed image. FIG. 9A and FIG. 9B show luminance signal levels in the vertical axis with respect to respective image data of color components R,G and B forming orange image data. The maximum value of luminance signal levels is 255. The exposures in the image data shown in FIG. 9A and FIG. 9B are different from one another, and image data shown in FIG. 9A is the image data of an image exposed in time shorter than an image shown in FIG. 9B.
In the image data shown in FIG. 9A, a value of image data of the B component in respective image data of R, G and B is approximately 10% of 255. In such case, noise and the like greatly affect the value of image data of B component, therefore, it is difficult to obtain sufficient reliability. On the other hand, in the image data shown in FIG. 9B, values of images of the R component and the G component in respective image data of R, G and B exceed the maximum value and saturated. Accordingly, it is difficult to obtain sufficient reliability concerning whether the hue of the image data shown in FIG. 9B is appropriate or not.
Accordingly, it is difficult to generate a composed image which reproduces the hue of the image appropriately by switching image data having different exposures shown in FIG. 9A and FIG. 9B in any manner in the related art.